The types of memory include volatile memories, which lose their information if their electricity supply is cut, and nonvolatile memories, which keep the information in the event of a power loss. Volatile memories include dynamic random-access memories (DRAMs), and static random-access memories (SRAMs).
A DRAM dynamic memory cell comprises an access transistor and a storage capacitor. Because of the existence of a leakage current in the insulator, the capacitor of the DRAM memory finally discharges, leading to a decrease in its voltage, and at the end of a few milliseconds the stored charge is almost entirely lost.
In order to avoid information losses in a DRAM, it is consequently necessary to recharge the capacitor so as to refresh the memory cell. An SRAM static memory cell comprises a bistable formed by two cross-coupled inverters and storing the datum, as well as two access transistors. SRAM memories do not need to be refreshed in order to keep the datum, but occupy more space than DRAM memories. For nonvolatile memories, EEPROM memories (Electrically Erasable and Programmable Read-Only Memory) may be mentioned.